The present invention relates to a rotary vacuum pump such as a turbo molecular pump or a molecular drag pump, a vacuum device provided with the rotary vacuum pump, and a pump connection structure.
A turbo molecular pump, which is used for high-vacuum evacuation, comprises plural stages of rotating fins and plural stages of fixed fins, which are placed alternately. Each rotating fin and fixed fin comprises plural turbine blades. The rotating fins are formed on a rotor which is driven by a motor, and the fixed fins are fixed to a base of the pump. A turbo molecular pump, which has a drag pump section in addition to the above-described turbine blades, is known. The drag pump section has a cylindrical part which is formed on a lower part of the rotor, and a screw groove stator which is provided near the cylindrical part.
In such a turbo molecular pump, the rotor on which the turbine blades and the cylindrical part are formed rotates at a high speed of several tens of thousands of rpm. If it is subject to abnormal external disturbance, there is a concern that the rotor and the stator side (for example, screw groove stator) may contact. In that case, a large impact may be applied to the stator side. Also, the rotor which rotates at high speed is normally subject to a large centrifugal force. Accordingly, there is a concern that the rotor may be broken when the rotor and the stator contact or the pump is operated continuously under conditions exceeding design limits. In such cases, there is a problem that a larger impact is applied to the stator, and a great shear force is applied to bolts that fasten the pump casing to the device main body.
A pump with bolt holes having plural steps expanding outwardly is known (for example, see Patent Document 1), so that a shear force is not concentrated in one place, thereby preventing breakage of bolts.
Patent Document 1: Japanese Patent Publication (Kokai) No. 2003-148388
In order to form the bolt holes having the plural steps expanding outwardly, a machining process becomes complex, thereby increasing cost. In the conventional pump, the bolts contact side faces of the stepped holes and elastically deform to absorb an impact force. However, due to the stepped hole, it is difficult to obtain sufficient elastic deformation.
The present invention has been made to obviate the above problems, and an object of the invention is to provide an improved rotary vacuum pump, a vacuum device provided with the rotary vacuum pump, and an improved pump connection structure.
Further objects and advantages of the invention will be apparent from the following description of the invention.